This invention relates to defibrillators and, more particularly, to defibrillators capable of compensating for patient resistance.
Most conventional defibrillators work on the principle of charging a capacitor to a predetermined voltage that corresponds to a desired energy level, and then discharging the capacitor through electrodes into the patient. Since the energy stored in the capacitor is proportional to the square of the capacitor voltage, the energy available to be delivered can be easily controlled in this manner
Shaping the waveform is currently accomplished by one of two methods. In the case of the damped sine wave (DSW) defibrillator, an inductor is placed between the capacitor and the electrodes, which creates a damped RLC circuit when connected to a patient and generates a damped sine wave. The base frequency and damping factor can be changed by changing the capacitor or inductance value and are also dependent on the resistance of the patient. In the case of the truncated exponential defibrillator, the time constant of decay of the waveform is adjusted by adjusting capacitor values and is also affected by patient resistance. The waveform can also be temporally shaped through the selected time of truncation. Additionally, the polarity of the connection between capacitor and patient can be reversed during delivery of the waveform to generate a biphasic, bidirectional waveform.
Variability of patient transthoracic resistance presents a problem in the design of defibrillator waveforms. Changes in patient resistance can affect many of the waveform parameters, including waveform size, shape and duration, and as a result can alter the efficacy of the delivered shocks. As a result, defibrillator manufacturers have developed methods for compensating for patient resistance. The notion of designing a defibrillator that is less affected by patient resistance becomes especially important when considering the use of defibrillators out of the hospital in less controlled environments. With the advent of public access defibrillation, a larger percentage of defibrillators will operate out of the hospital. In particular, a recent area of focus in the defibrillation industry has been the automatic external defibrillator (AED). The AED functions like an implantable defibrillator in that it automatically identifies fibrillation and then delivers corrective therapy. The device allows non-medical professionals to resuscitate ventricular fibrillation victims. In such a setting, where it may be impossible for the operator to even estimate an individual""s transthoracic resistance, it would naturally be desirable to have a defibrillator that is capable of delivering an effective amount of energy to any individual in a population with a wide range of resistances.
A basic approach to delivering a desired current to a patient is to determine the patient resistance and then deliver a shock with a peak voltage that corresponds to the desired current according to Ohm""s law. One such method involves measurement of the subject resistance, before delivering the shock, by passing a low-intensity, high frequency current through the defibrillation electrodes on the subject. This was demonstrated by Geddes et al. as reported in a paper entitled xe2x80x9cThe Prediction of the Impedance of the Thorax to Defibrillating Current,xe2x80x9d Medical Instrumentation, vol. 10, pp. 159-162, 1976. A limitation of this method is that the patient resistance changes with successive shocks and is also affected by the intensity of the shock. Thus, this method gives only an estimate of the electrode resistance at the time of the delivered shock. Several patents have been issued that relate to this method:
A technique requiring pre-shock measurement of patient resistance is suggested in U.S. Pat. No. 5,725,560 to Brink for use in scaling a reference waveform having the shape desired for the defibrillation waveform applied to the patient. Brink notes that the defibrillation waveform can be characterized in terms of voltage, current, instantaneous power, or total energy. A closed-loop technique is described with emphasis on current control.
Another method involves delivery of a first shock and measurement of resistance during the shock. The resistance can be determined by measuring delivered voltage and current and using Ohm""s law, or it can be determined through the measurement of the RC time constant in the case of a truncated exponential defibrillator where the capacitance C of the defibrillator is known. An example of this method is disclosed in U.S. Pat. No. 4,328,808 to Charbonnier et al. The disclosed device measures peak current and peak voltage and computes resistance using Ohm""s law. U.S. Pat. No. 5,733,310 to Lopin et al. describes a method of determining resistance from a shock that is well below threshold levels. The resistance value is used to determine how high to charge the defibrillator capacitance for the subsequent therapeutic shock. Determining resistance from a prior shock involves delivering an extra shock and thus is undesirable. It also can be inaccurate. Geddes et al. have shown that transthoracic resistance decreases with successive defibrillation shocks, which suggests that the resistance measured during the pre-therapy shock may be different than the resistance during therapy. Geddes et al., xe2x80x9cThe decrease in transthoracic impedance during successive ventricular defibrillation trials,xe2x80x9d Medical Instrumentation, vol. 9, pp. 179-180, 1975.
An additional method for compensating for subject resistance, applicable to truncated exponential defibrillators, uses a fixed tilt waveform. Waveform tilt is defined as the amount the voltage decays between the start and truncation of the waveform:   tilt  =                    V        i            -              V        f                    V      i      
where Vi is the initial voltage of the waveform and Vf is the final voltage. With fixed-tilt waveforms, the voltage on the storage capacitor of the defibrillator is monitored during the delivery of the shock. The shock is terminated when this measured voltage reaches a pre-set value. Thus the storage capacitor always discharges the same amount of energy independent of the patient resistance. Variations of the fixed-tilt method can be found in both implanted and external truncated exponential defibrillators. For example, U.S. Pat. No. 5,593,427 to Gliner et al. suggests the use of fixed tilt with an upper or lower limit on pulse duration. The disadvantage of the fixed-tilt method is that duration is variable and thus cannot be optimized.
Another problem with conventional defibrillators is that fundamental changes to the waveform shape cannot be made. For example, conventional damped sine wave defibrillators cannot be adjusted to deliver a truncated exponential waveform. A biphasic, bidirectional truncated exponential waveform has been shown to have advantages over a damped sine wave, including defibrillation with less energy and fewer electrophysiologic side effects. DSW defibrillators are the prevalent type of defibrillator in use in hospitals today, and they may soon become obsolete due to the improved waveform mentioned above. Therefore, a need exists for a way to adapt existing defibrillators for use in defibrillation with such a new waveform or others.
The present invention provides, as a first aspect thereof, a controlled-power defibrillator for automatically compensating for changes in patient resistance during delivery of a defibrillation pulse. The controlled-power defibrillator comprises an energy storage device, a pair of electrodes adapted for connection to a patient""s body, a power converter having an input connected to the energy storage device and an output connected to the electrodes, and a closed-loop power control circuit operatively connected to the power converter for controlling the power delivered to the patient. The power control circuit includes means for generating a control signal having a defined relationship with a desired delivered power waveform, means for monitoring a parameter having a defined relationship with actual delivered power, and means for adjusting the delivery of power to the patient as a function of the difference between desired power and actual power.
This permits delivery of a waveform having a desired shape, duration and total delivered energy independent of patient resistance, unlike known fixed-tilt methods and others. In its preferred embodiment, the defibrillator controls power by controlling the rate of decay of the voltage on a storage capacitor, which is the preferred form of energy storage device. The control circuit generates a desired capacitor voltage signal as a control signal for this purpose. This desired voltage signal is not the same as the voltage desired to be applied to the patient but has a defined relationship with the desired power waveform delivered to the patient.
According to another aspect of the present invention, a waveform adapter is provided for use with an existing defibrillator capable of generating a given waveform. The waveform adapter is capable of capturing energy from the discharge of the existing defibrillator, converting captured energy to a second waveform and delivering converted energy through a pair of electrodes to a patient.
It is a general object of the present invention to provide improved methods and apparatus for defibrillation.
Another object is to provide a method of defibrillation that is less affected by transthoracic resistance variability than known fixed-tilt methods and other conventional methods of defibrillation.
A further object is to provide a waveform adapter for use with existing DSW defibrillators and other defibrillators designed to deliver a single type of waveform, whereby the energy from the existing defibrillator may be captured and converted to a different waveform, thus extending the useful life of existing defibrillators.
These and other objects and advantages of the present invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings.